Change value of required attribute from "true" to "required"

Love this module!

I have a request for a minor change. When the module adds the required attribute to required form fields, it sets it to “true” which is incorrect. According to W3C guidelines:

“The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value. The values “true” and “false” are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.”

This means the value for the required attribute either shouldn’t be set or if it is, it’s set to “required” instead of “true”.

I'll attach a patch below.